For two decades, a loose-knit group that includes some of the country's wealthiest people has underwritten the political push for school choice, promoting ballot initiatives and candidates who favor competition for traditional public schools.

But when a member of this elite group was elevated to education secretary, the appointment opened a philosophical schism that now threatens to shatter the alliance, turn billionaires against each other and possibly lead some school-choice advocates to join with teachers' unions, their archenemies.

Fueling the split is the anticipation of a plan from President Donald Trump's administration that could offer parents federal dollars to send their children to private schools, including religious and for-profit institutions.

"As much as we are aligned on change, we aren't always aligned on how much change or how. Sometimes we fight," said Derrell Bradford, executive vice president of the school-reform group 50CAN.

The movement has been cleaved into two camps: those who want to use choice to improve public schools and others, like Education Secretary Betsy DeVos, who want to go further by allowing tax money to flow to private schools through vouchers, government-funded scholarships or corporate tax credits.

The differences that once seemed minor are at the heart of a potential seismic shift in the school-choice movement.

School-choice programs were first proposed in the 1950s by Nobel Prize-winning free-market economist Milton Friedman. Since the beginning of this century, they have grown quickly, although the overwhelming majority of students still attend traditional public schools.

Thirty states plus Washington, D.C., have some combination of vouchers, government-subsidized education savings accounts or tax credits that help families afford private-school tuition or encourage private groups to fund scholarships, according to EdChoice, an advocacy group founded by Friedman and his wife.

Still, less than 1 percent of children in kindergarten through high school used vouchers to attend private schools in 2015. Just 5 percent of students were in charter schools that year, when charters were operating in more than 40 states. That's up from about 3 percent in 2008, according to the Department of Education.

Charter schools are public but in several states are not held to the same accountability standards as traditional schools, which in theory gives them more freedom to innovate.

When standardized test scores of children who switched to charter schools or used vouchers are compared with those of students who remained in traditional public schools, some results have been promising. Other studies have shown little effect or even worse outcomes.

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THE BIGGEST DONORS

Wal-Mart's Walton family, Los Angeles billionaire Eli Broad and the father of Microsoft founder Bill Gates have been some of the largest political contributors to the school-reform movement, according to an analysis by The Associated Press of campaign-finance records compiled by the National Institute on Money in State Politics and state campaign-finance regulators.

Microsoft's Bill Gates avoids political contributions. His family's massive foundation - which his father, Bill Gates Sr., co-chairs - is a major contributor to education-reform nonprofits. The elder Gates contributes personally as well.

In the past, contributions have flowed to reform-minded groups regardless of whether they supported public charter schools, vouchers for private schools or a combination. Now those funders are beginning to split into competing camps.

Groups such as Stand for Children, which pushes for charter schools among other educational changes, are being more vocal now because "the level of ambition of the efforts to expand vouchers is so high," said Jonah Edelman, co-founder and chief executive, who called the voucher proposals "a dramatic effort to undermine public education."

Edelman, whose organization receives part of its funding from Walton, Broad and Gates-related charities and family members, said his group will fight any administration effort to promote vouchers nationwide.

Stand for Children has worked with teachers unions in the past on state-level measures to provide more money for schools and raise teacher pay. But its calls for more charters often put it at odds with the unions, which are powerful political entities in many states.

The American Federation of Teachers has had conversations with some charter school advocates it has opposed in the past about coordinating to fight a national voucher program, said Randi Weingarten, president of the union.

To identify those who have shaped the school-choice movement across the country, the AP analyzed the political contributions of 48 individuals and couples who have given at least $100,000 to related statewide ballot measure campaigns. That group accounted for the majority of reported contributions to campaigns favoring school-choice initiatives from 2000 through last year. The donors nearly matched all the measures' opponents.

The group also wields political clout in addition to school-related ballot initiatives.

The AP analysis showed that the same wealthy funders contributed a total of more than $200 million from 2007 through last year to candidates and political action committees, some of which are supporters of school choice.

The funders are from a variety of industries and regions, with a concentration of investors and tech innovators from Seattle and Silicon Valley. Some give millions more to research centers, legal funds, foundations, charter schools and scholarships that promote pieces of the cause.

Political campaigns often are a clash of titans. The nation's two largest teachers' unions - the National Education Association and American Federation of Teachers, which have 4.5 million members combined - contributed $573 million to campaigns and political action committees over the same 10-year period, much of it designed to counter the spending of the wealthy families and their affiliated groups.

Both unions and school-choice advocates also made independent political expenditures, which are impossible to tally because the details are not required to be reported in many states.

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THE HEART OF THE DEBATE

For the past decade, school-choice supporters have split over the merits of private-school vouchers versus public charter schools, according to William Oberndorf, a San Francisco investor, major contributor to the movement and chairman of the American Federation for Children, which was led by DeVos until last year.

He urged all school-choice advocates to drop their divisions and support the core belief that choice in any form is a good thing if it lets parents make decisions about what is best for their kids.

"Most Americans have that already," Oberndorf said in an interview. "They have that if they have enough money to move to a good school district or send their kids to private school. It's really only poor people who don't have choices."

Some of the wealthy school-choice funders are not swayed and object to being grouped together with supporters of private-school vouchers.

Nick Hanauer, a Seattle investor who had an early stake in Amazon and who spent $1 million in 2012 to support a successful ballot measure to allow charters in Washington state, called vouchers "a stupid idea." Consider that in the most elite private schools across the country, tuition runs $30,000 to $40,000 a year, he said.

"No voucher program is a $40,000-per-year program. It's a $10,000-per-year program, or a $6,000-per-year program, which means that voucher advocates aren't serious about delivering high-quality education," he said. "They're simply interested in making it cheaper for people who already send their kids to private school to continue to do this."

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DeVOS DEEPENS DIVIDE

The schism between wealthy funders over private-school vouchers became visible earlier this year during the confirmation process for DeVos, a billionaire heiress who was nominated by Trump for education secretary despite having no experience as a teacher or school administrator. DeVos attended private religious schools in Michigan. Her children were homeschooled and attended religious schools.

Broad, a charter school advocate who has contributed at least $3.7 million since 2007 to political efforts to expand charters, wrote a letter to senators urging them to reject DeVos, calling her "unprepared and unqualified."

With DeVos at the helm of the Education Department, "much of the good work that has been accomplished to improve public education for all of America's children could be undone," wrote Broad, who declined to be interviewed by the AP.

DeVos was confirmed only when Vice President Mike Pence cast the vote to break a 50-50 tie in the Senate.

Overstock founder Patrick Byrne, who spent nearly $3 million on a failed 2007 Utah bond measure to allow charters, said DeVos was an "A-plus." Venture capitalist Timothy Draper, who put $23 million behind a failed attempt to bring vouches to California in 2000, also had high praise for her.

"Having someone who really understands choice in education and how helpful that can be ... is going to be good for education," Draper said.

The sides have been at odds previously, including in Arizona, where in April lawmakers passed and the governor signed a law to expand a voucher program. DeVos's American Federation for Children made a total of $500,000 in independent political expenditures in 2014 and 2016. The anti-voucher Stand for Children spent $1 million in the state during that span.

The Trump administration has not released any details about a voucher plan. But early this month, Trump asked lawmakers to work with him to extend school choice to millions more children, including low-income Hispanics and blacks "who deserve the same chance as every other child to live out their dreams and fill up their hearts and be educated at the top, top level."

The outline in his budget proposal calls for $1.4 billion in school-choice initiatives, including $250 million to launch a voucher program. That money would come in part from cutting $9 billion from K-12 education, including teacher training and after-school and summer programs.

The Trump budget calls for eventually increasing annual school-choice funding by $20 billion.

In her first major policy address as education secretary, DeVos said allowing parents to decide where their children will be educated is essential, whether they choose homeschooling, traditional schools, magnets, charters, online schools or private schools.

"I think that the first and most important measure of accountability to the public is to the parents in general, right?" said DeVos.